Absorption spectra of Ti-doped GaAs

We observed the 3$d^1$ intracenter transition ${}_{}{}^2{}_{}{}^{}\mathrm{}_{}^2{}_{}{}^{}$ ${\mathrm{T}}_2$ in semiconductor crystals having a tetrahedral crystal field. This absorption was measured in Ti-doped melt-grown GaAs crystals. The spectrum consists of two sharp zero-phonon lines at 0.566 and 0.569 eV and of some broad phonon features with a maximum at 0.64 eV. The ${\mathrm{Ti}}^{3+}$(3${\mathrm{d}}^1$) spectrum can only be observed in high-resistivity GaAs crystals. In n-type conducting crystals the compensation of titanium acceptors leads to the appearance of the ${\mathrm{Ti}}^{2+}$(3${\mathrm{d}}^2$) intracenter transitions at 0.66 and 1.01 eV. The corresponding single acceptor level ${\mathrm{Ti}}^{3+}$/${\mathrm{Ti}}^{2+}$ was identified to be at 0.23±0.01 eV below the conduction band on the basis of deep-level transient spectroscopy and capacitance transient measurements.